They organise various national championships for older bikes & riders. I ride in the over 50s which allows me to ride a pre 1990 "evo" model and for that I restored a Maico 500 1986

However I've been trying to get an entry to the annual Vets MX des Nations event at Farleigh Castle (Chuck Sun, Mike Bell, Ryan Hughes, Doug Dubach, Jeff Ward and Ron Lechien all made the trip across this year) and also want to do selected National Twinshock Championship events, especially at Hawkstone Park. This is an English based championship and their rules mean I need to ride a twinshock machine.TWINSHOCK. ANY BIKE MANUFACTURED WITH TWO REAR SHOCK ABSORBERS AND WITH DRUM BRAKES AND AIR COOLED INCLUDING UPTO 1984 HUSQVARNAS, PRE 82 MONOSHOCK YAMAHAS AND WATER COOLED ORIGINALS THAT WERE MANUFACTURED PRE 1982. FORKS MUST NOT BE OF GREATER DIAMETER THAN 46MM.

http://nationaltwinshock.co.uk/

So a couple of weeks ago I bought a Yamaha YZ465 imported from California with a view to restoring and racing it in these twinshock events. I have it all stripped down and it is in pretty good shape. The exhaust and the suspension need some work, but not too bad and the rest is just tidying up.

Kevin, Not to hijack your thread but I picked this little number up from E-bay this week. I wasn't interested so much in the arm but the clutch extender that came with it. Not as sexy looking as the CNC versions out today but the price was right.

These became real popular in the 80's to reduce clutch pull effort and there were several designs out (such as EZ-Pull) which fit most bikes but had so much slop that it was a pain to keep adjusted. That was a problem with these, yes it made the clutch pull lighter but the clutch adjustment range became very sensitive. This clutch arm was for an IT465 and was in slightly better shape than my YZ clutch arm.

Here is a picture of the differance in arm length:

Plus I think the IT arm is a little beefier.

Five minute deal to drop in place.

Turned my four finger clutch pull into a two finger pull. These extenders go for as much as $40 but the arm with attached extender was around $25 shipped. Nice.

OK, fair warning, I am really going to beat a dead horse in this section!

------------------ Top end continued

During the time the cases and top end were apart, I made some effort to consider how the Yamaha OW factory porting methods (shown earlier) could be applied to the YZ465 / 490. Of course this would be for future modifications to another motor as the top end of the current motor has already gone back together.

This is interesting to me because many of the methods used in the factory OW’s were closely held secrets back then. So it is fascinating now to be able to look back at what was done and how some of the same methods have turned up in later production race bikes. In fact, I found several photos of big bore MX cylinders and competition snowmobiles that will be included to illustrate this point.

In any case, two questions came to mind after studying the photos of the factory OW cylinder photos and how they may relate to modifications of the 465.

1) Can the two additional intake to transfer ports be added to the 465 cylinder?

- We already know the answer is yes as LOP cylinder shows this. Still I think it is worth taking a closer look especially when comparing these early implementations to those in modern big bore cylinders.

2) Can the OW style case induction port be added?

- To answer this we can look at the cylinder to case interface to consider what is possible.

To refresh, here again is the OW cylinder where the new ports in question have been labeled.

New intake/transfer ports

The next photo better compares the OW cylinder to the LOP 465 cylinder where the top row is the OW cylinder and the bottom is the LOP.

The red arrows show the corresponding intake to transfer ports. The blue arrow points to the case induction port in the OW cylinder and where we would like it to be in the 465 cylinder.

The OW cylinder is thought to be from a late ‘70s (pre-watercooling) 250. The LOP 465 photos (bottom row) show the addition of the intake/transfer ports like the OW. I was later to find out that these are called "Boyesen Ports" after inventor Eyvind Boyesen of dual stage reed valve fame.

It turns out the ports in both these cylinders are relatively small compared to photos of newer model production cylinders like CR500 and others that I have found. Unfortunately, making these ports much larger in the 465, without breaking through the side, would be difficult due to the location of the cylinder studs and the clearance needed for the stud nuts.

Perhaps LOP has defined the limit on this?

Case InductionRegarding the case induction port question, the first thing I tried to do was find another example of this, especially one with photos showing the engine case side as I do not have a photo of the OW engine case. I was lucky to find this example,

The left photo above shows the induction port on the engine case side. It is milled to some depth below the base gasket surface.

The right photo above shows the base of the cylinder. The induction port is milled from the base right through to the intake port. Notice too how the intake side of the sleeve is cut back near the induction port.

This cylinder also has the LOP/OW style intake to transfer ports (although they appear much larger). I believe this is a good example of how to go about adding the case induction port to the 465 motor.

----

Now let’s look at the YZ465 case to cylinder interface where the cylinder to case induction port would be. The upper left picture shows the area in question and where any modification would take place.

On the case side, there is danger of milling through the crank case and into the transmission cavity (very bad!). A possible region for milling of the case and cylinder is shown in the lower left photo. This region, in green, leaves just enough surface area in the case for a good seal when the case halves come together.

The above right photo shows how the sleeve might be cut away. Only half the region is colored in green (along the case centerline) so the width of the cutout would twice shown.Note that the sleeve cut away is in an area corresponding to the skirt cut away in the piston. Cutting the sleeve in this area does not reduce support for the piston skirt.

So, given the above, it does appear that both OW style port modifications could be applied to the 465.

Take a break now with a few photos. Here is Lauren Offner of LOP on left,

I have a 1980 YZ 465 and I have a REALLY nice set of gold rims to put on it, but when I went to put them on my break hub won't fit on the gold rims (about a 6 1/4" hub) and my gold rim is a 5" hub. - I think the gold rims fit a 1981 YZ 465 (which is a 5" hub!) Could you tell me if I am right or wrong on that? and also the axle pin is bigger on the gold rim Thank you! ;)

I have a 1980 YZ 465 and I have a REALLY nice set of gold rims to put on it, but when I went to put them on my break hub won't fit on the gold rims (about a 6 1/4" hub) and my gold rim is a 5" hub. - I think the gold rims fit a 1981 YZ 465 (which is a 5" hub!) Could you tell me if I am right or wrong on that? and also the axle pin is bigger on the gold rim Thank you! ;)

~Jenn

Welcome and congratulations on you ownership of an '80 465. I am assuming you are talking about the rear wheel?

Wait here while I take some measurements....

ok, I am back. The 465/490 rear hubs measure about 5 7/8 inches diameter across the steel brake drum. I am not sure where you are taking your measurements but try comparing to that.

You may want to verify the stamping on your rim to make sure it is not for a YZ125 or 250. The YZ465/490's should be stamped 18 x 2.50.

On the axle diameter. The 1981 and later axles are larger diameter. I fitted a yz 490 rear wheel into a 1980 yz 465 swing arm but it required milling the axle slot to accept the larger axle spacers etc. You have do something about the brake plate too. If you look back a few pages you will see the modifications I did to mine. Should be right around here http://advrider.com/forums/showthrea...=772033&page=2,